A 4-pair cable called a category 5 cable is generally used for high-speed LAN systems with transmission signal speeds up to 100 Mbps. Specification standards for characteristic impedance, near-end cross talk and attenuation of this cable are specified in ISO/IEC and EIA/TIA, so that performance is ensured by conformity to the standard.
Accordingly, conformity to the standard is achieved by differentiating the twist pitches of four twisted wire pairs (T, T, T, T) as shown in FIG. 1, with the difference between the twist pitches being made as large as possible. Each twisted wire pair T is made by twisting a pair of insulated wires 1 each consisting of an electrically conductive wire such as soft copper wire covered by polyolefin thermoplastic resin.
A communication cable is made in a configuration such that a interposer 2 is disposed at the center of the entwined twisted wirepairs (T, T, T, T), as shown FIG. 2, and/or a support winding (not shown) is provided around the entire twisted wire pairs (T, T, T, T) to prevent the twisted condition from loosening, thereby ensuring the geometrical stability of the arrangement of the twisted wire pairs (T, T, T. T). In FIG. 2, reference numeral 3 denotes a sheath made of a thermoplastic resin.
Recently, a technique has been proposed to separate the twisted wire pairs (T, T, T, T) by means of a cross-shaped interposer 2 as shown in FIG. 3.
However, although the requirements of the standards of category 5 can be met sufficiently with the technologies of the prior art, a yet further upgraded cable performance has been proposed for application to the giga-bit Ethernet, which requires a yet further lower level of near-end cross talk. There is also a demand to decrease the difference in the signal propagation delay time between the four pairs of twisted wires. In order to reduce the near-end cross talk in the prior art, twist pitches among the pairs are different. However, this increases the difference in delay time and therefore the two items of target characteristics cannot be satisfied with only this technique.
Performance requirements for cables used for Gbit Ethernet are currently discussed at TIA TR41. Although in a draft state of at present, a category 6 (Cat.6) standard is proposed for UTP (Unshielded Twisted wire Pair) having a transmission capability of up to 250 MHz. On the other hand, Anixter Inc. of the U.S.A. separately specifies a level 7 of performance that is equivalent to Cat.6, for the characteristics of a channel combining a cable and connectors as a specification standard. The category 6 (level 7) standard requires that the attenuation is lower than that of category 5 by 12 dB.
In order to meet the requirements of category 6 and level 7 of Anixter Inc., it is necessary to keep the difference between the maximum and minimum values of delay time among the four twisted wire pairs constituting the cable within 25 ns/100 m. In the case where the twist pitches are differentiated among the twisted wire pairs so that the requirement of Cat.6 for the reduction of near-end cross talk is reliably met with the prior art, the difference in delay time however becomes larger than 25 ns/100 m.
Also it may be considered possible to reduce the cross talk even with such twist pitches that keep the difference in delay time within 25 ns/100 m for the four twisted wire pairs (T, T, T, T) (insulated wires with an outer diameter of insulation in a range from 0.92 to 0.96 mm twisted with pitches from 10 mm to 18 mm, based on experience), by separating the twisted wire pairs with the cross-shaped interposer 2 as shown in FIG. 3. In order to meet the requirement for reducing the near-end cross talk proposed by the Cat.6 draft with a sufficient margin, it is necessary to separate the twisted wire pairs with a sufficiently large space by means of the cross-shaped interposer that has a sufficient thickness, thus resulting in a sturdy cable structure where arrangement of the four twisted wire pairs (T, T, T, T) can be firmly maintained. However, it is difficult to insert the cable covered by the sheath 3, while keeping the cable flat, into a modular plug 4 to have the cable 5 held by the modular plug 4 with a sufficient force, as shown in FIG. 4, because the cable including the cross-shaped interposer cannot be easily flattened and is difficult to insert into the modular plug 4.